The question of why exactly AR-15s are so popular is doubtless complex, with different people being drawn for different reasons. With over ten million AR-15s in civilian hands (though given the number of owners I run into on gun boards who own 3+ this doubtless represents well under ten million Americans who actually own one) and the number of homicides committed nationwide using rifles of any kind in the hundreds, mayhem is clearly not the only motive.
One of the things which particularly upsets critics about the AR-15 as a rifle for civilians is its military origin. While the civilian AR lacks the fully automatic mode which military M-4 Carbines have (fully automatic mode allows the rifle to fire continuously as long as the trigger is pulled down -- at least for the three seconds or so it would take for the rifle to empty its standard thirty round magazine), it is a semi-automatic weapon and thus can fire as quickly as the shooter can pull the trigger. Other less notorious semi-automatic rifles, shotguns, and pistols can of course do exactly the same thing (which gun advocates are quick to point out when proposals to ban "military style" weapons are put forward) but the AR-15 and its relatives are the ones which have the appearance and reputation to make them look particularly alarming to gun control advocates in this respect.
The same military pedigree is, of course, one of the attractions for many AR owners and shooters. As critics have pointed out at some length, many advertisements for these "tactical" types of guns make appeals to testosterone in one way:
While "tacti-cool" looks are certainly a factor in the AR's popularity, as is the design's connection with America's military, I'd argue that there's also an interesting intellectual property and innovation explanation for the popularity of the AR platform.
The last phrase there, "AR platform" is the key, because the "AR-15" is not really just one model of gun. The trademark for the term "AR-15" is owned by Colt, which purchased the rights to the trademark from Armalite decades ago. However, the term is used far more generally to refer to any civilian rifle built around the specifications of the M-16/M-4 weapons system. Since the "mil spec" design is not protected by any patent, hundreds of manufacturers, large and small, have a standard platform for which they can built complete rifles or just specialized parts. The gun rack at a store might contain AR-15s ranging from $500 to $3000 in price. Some are light "tactical carbines" with military style 14.5" barrels made legal by a permanently attached flash hider or muzzle brake which takes the total barrel length out to 16", some are target rifles with heavy 20" or 24" competition barrels. Some have plastic molded handgrips, others have aluminum handguards with rails for mounting accessories.
Hobbyists often assemble their own rifles from parts (as, full disclosure, I am myself currently engaged in doing) and in doing so they can put together a "franken gun" with favorite (or most affordable) parts from a variety of manufacturers, and do so in the confidence that a lower receiver from one manufacturer, an upper receiver from another, a barrel from the third, and a handguard from yet another will all fit together perfectly and function as a whole, because they are all built to fit together according to the same basic military specifications that are available to all.
|An AR-15 can be assembled using swappable parts from many manufacturers.|
The standardization of this mil spec design allows guns very, very different from any military rifle to be made primarily with AR components. Swapping out the barrel and upper receiver can convert the rifle to shoot an entirely different cartridge from the military standard .223. Several manufacturers even produce parts which can replace a few parts to turn an AR into a precision bolt action rifle.
Guns are durable goods (my other four rifles are all over fifty years old and still function just fine) and one of the things which people consider in selecting a rifle is whether parts and ammunition for it will continue to be available in the future. In general, this has led to rifles having a fairly low rate of technical change and a fairly small number of major manufacturers. If you buy a rifle from Winchester, Remington, Marlin, or Ruger, it's a good bet that those same manufacturers, and often that very same model of rifle, will still be around in 20 or even 50 years. (Among top hunting rifles, the Winchester Model 70 bolt action was introduced in 1936 while the Remington Model 700 has been in production since 1962.) A shooter who purchases one of their rifles can expect to be able to get parts and service for it for decades to come. But what of some small startup company offering an innovative design? Will they even be around in ten years?
Perhaps due to such questions, for many years the top selling rifles were usually from the same big, old companies. The AR platform removes such worries, however. Buy an AR put out by some small start up with interesting new technology, and even if they aren't around in a few years there are dozens of other companies selling compatible parts. The result has been a boom in small companies making parts which AR hobbyists find exciting, and far more variety and innovation than is normally seen in the firearms industry.
It seems clear to me that a good share of the reason for this is that the "mil spec" guidelines for how an AR has to be designed, guidelines which make all mil spec parts compatible, created an environment in which innovation could thrive. And yet, the context for this innovation was a specification designed by a government agency: the department of defense in collaboration with manufacturers such as Armalite and Colt.
In the software world, this is the kind of situation that developers work hard to achieve. You want enough different people to work on a software platform that there are lots of programs that can work on the same platform and build on each other's strengths. If everyone builds their own proprietary platform, there's much less innovation. But getting the common platform takes a lot of work. There's a value to everyone in having the commonly shared technology for everyone to work off of, but getting there involves both getting people to share their work for free in some cases, and also keeping them from "branching" the project -- adding their own cool features which are not compatible with other aspects of the platform.
In this case, it's the set guidelines of the military specification and the tremendous prestige which the mil spec design has with gun buyers (the idea that the military design represents durability and effectiveness as well as the ability to fit together with other mil spec parts) which minimizes "branching" within the AR world. Manufacturers will innovate within a specific area (say a competition grade trigger assembly with a unique design or a chamber designed to make extraction of the spent shell more smooth and easy) but if they fail to produce parts that still work seamlessly with other mil spec parts, buyers tend to reject the design.
I'm not sure how this approach could be applied to other areas. This may be a case in which the prestige of the open design platform has a unique hold on customers which would be hard to reproduce elsewhere. But it does, I think, help explain the unique hold which the AR platform currently has on American gun hobbyists. The massive variety of rifles and customization options that have sprung up have simply made being an AR hobbyist far more interesting than focusing on virtually any other rifle. Indeed, the only platform which has even close to the amount of customization and variety which the AR does is a much older military design: the M 1911 .45 pistol.